1. Field of the Invention
This invention relates to the field of packaging semiconductor devices, and more particularly to attaching a die to a package.
2. Description of the Prior Art
In packaging of integrated circuit devices, as well as discrete semiconductor devices, one of the major problems involved is providing good thermal bonding between the die and the package to which the die is attached. A commonly used method of attaching a die to a package is to coat the surface of the package to which the die is to be attached with a metallic composition, commonly referred to as a preform, which, when heated, melts and causes the die to adhere to the package. FIG. 4 illustrates a section taken through a portion of a GaAs device packaged by the foregoing process. As will be appreciated by reference to FIG. 4, a major problem in this type of die attachment process is that voids 12 are formed, that is incomplete bonding occurs between preform 13 and the adjacent side of the die 14 occurs, for example, by the inclusion of air. Also, gases may be trapped in the preform, resulting in voids 15 within the preform 13. Such incomplete bonding prevents good heat flow from die 16 to package 17 and causes what is referred to as "hot spots". More particularly, good heat transfer does not occur where the air is trapped between the preform material and the adjacent surface of the die and in particular in connection with high power devices catastrophic failure may occur. Such problem is especially acute for power GaAs microwave monolithic integrated circuits where it is not uncommon to have two watts of DC power dissipated for one watt of output power from such device. One of the common causes for voids is the out-gasing of the preform alloy during the die attachment and the subsequent trapping of air bubbles.
The task of successfully attaching a GaAs microwave monolithic integrated circuit to a package is further complicated because that type of device includes a backside via hole 17 which is opened to the field effect transistor source on the front side of die 16. This hole is generally plated, but not planarized, and accordingly can easily trap air during the die attachment, forming not only a void at the via, but also affecting the quality of the die attachment in the vicinity of the via hole area.
Prior art attempts to minimize voids in the attachment of a die to a package have included choosing the preform material such that it has the characteristics of minimum out-gasing and tightly controlling the attachment process from a temperature standpoint.